Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

Amiard et al. (1994) investigated the impact on soft tissue Pb concentrations of various
feeding regimes on oysters (Crassostrea gigas) during their spat rearing. They fed test groups of
C. gigas different amounts of Skeletonema costatum and additional natural phytoplankton grown
in test solutions. Results showed that size and food intake both negatively correlated with metal
concentrations in soft tissue. The authors hypothesized that this relationship was due in part to a
diluting effect of the food.
In summary, nutrients affect Pb toxicity in those aquatic organisms that have been studied.
Some nutrients seem to be capable of reducing toxicity, though the mechanisms have not been
well established. Exposure to Pb has not been shown to reduce nutrient uptake ability, though it
has been demonstrated that Pb exposure may lead to increased production and loss of organic
material (e.g., mucus and other complex organic ligands) (Capelo et al., 1993).
Interactions with Other Pollutants
Most of the scientific literature reviewed in this section considered how Pb and other
elements combine to affect uptake and exert toxicity. Research on the interactions of Pb with
complexing ligands and other physical and biological factors was more thoroughly discussed in
Section AX7.2.3.4. Predicting the response of organisms to mixtures of chemicals is difficult
(Norwood et al., 2003). There are two schools of thought on addressing chemical mixtures in
toxicology. The first focuses on the combined mode of action of the individual mixture
substances while the other focuses on the organism response to the mixture as being additive, or
some deviation from additive (synergistic or antagonistic). The mode of action model assumes
that each of the individual substances has similar pharmacokinetics as well as mode of action.
Approaches to modeling combined mode of action include: 1) joint independent action for
toxicants with different modes of action; and 2) joint similar mode of action. Within the
scientific literature the deviations from additivity approach can be confusing as some authors
report concentration additivity, while others report effect additivity. The concentration additivity
model assumes that the sum of the concentrations will result in a level of effect similar to the
simple sum of the effects observed if each chemical were applied separately (e.g., Herkovits and
Perez-Coll, 1991). Toxic units (TU) may be used to describe the concentration additivity results
for mixtures (e.g., Hagopian-Schlekat et al., 2001). Effects additivity suggests that the level of
effect of a mixture will be the sum of the effects of each chemical used separately. Thus, the
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